Chapter 36. Procedural Languages

PostgreSQL allows users to add new
programming languages to be available for writing functions and
procedures. These are called procedural
languages (PL). In the case of a function or trigger
procedure written in a procedural language, the database server has
no built-in knowledge about how to interpret the function's source
text. Instead, the task is passed to a special handler that knows
the details of the language. The handler could either do all the
work of parsing, syntax analysis, execution, etc. itself, or it
could serve as "glue" between
PostgreSQL and an existing implementation
of a programming language. The handler itself is a special
C language function compiled into a shared object and
loaded on demand.

Writing a handler for a new procedural language is described in
Chapter 47. Several procedural languages are
available in the standard PostgreSQL
distribution, which can serve as examples.

A procedural language must be "installed" into each
database where it is to be used. But procedural languages installed in
the database template1 are automatically available in all
subsequently created databases. So the database administrator can
decide which languages are available in which databases and can make
some languages available by default if he chooses.

For the languages supplied with the standard distribution, the
program createlang may be used to install the
language instead of carrying out the details by hand. For
example, to install the language
PL/pgSQL into the database
template1, use

createlang plpgsql template1

The manual procedure described below is only recommended for
installing custom languages that createlang
does not know about.

Manual Procedural Language Installation

A procedural language is installed in a database in three steps,
which must be carried out by a database superuser. The
createlang program automates step 2 and step 3.

The shared object for the language handler must be compiled and
installed into an appropriate library directory. This works in the same
way as building and installing modules with regular user-defined C
functions does; see Section 33.7.6.

The optional key word TRUSTED specifies that
ordinary database users that have no superuser privileges should
be allowed to use this language to create functions and trigger
procedures. Since PL functions are executed inside the database
server, the TRUSTED flag should only be given
for languages that do not allow access to database server
internals or the file system. The languages
PL/pgSQL,
PL/Tcl, and
PL/Perl
are considered trusted; the languages
PL/TclU,
PL/PerlU, and
PL/PythonU
are designed to provide unlimited functionality and should
not be marked trusted.

Example 36-1 shows how the manual
installation procedure would work with the language
PL/pgSQL.

Example 36-1. Manual Installation of PL/pgSQL

The following command tells the database server where to find the
shared object for the PL/pgSQL language's call handler function.

then defines that the previously declared call handler function
should be invoked for functions and trigger procedures where the
language attribute is plpgsql.

In a default PostgreSQL installation,
the handler for the PL/pgSQL language
is built and installed into the "library"
directory. If Tcl/Tk support is configured in, the handlers for
PL/Tcl and PL/TclU are also built and installed in the same
location. Likewise, the PL/Perl and PL/PerlU handlers are built
and installed if Perl support is configured, and PL/PythonU is
installed if Python support is configured.